1. Field of The Invention
The present invention relates to a detector which, for example, detects changes of incident light to a light receiving element for detecting a trespasser or the like in a specified detection area.
2. Description of The Related Art
Such a detector for crime prevention in prior art has a configuration shown in FIG. 10, for example. In FIG. 10, a detector 1 comprises a base 2, which is mounted on a ceiling, a wall or the like, a sensor section 3, which is mounted on a surface (bottom face) of the base 2, and a cover 4, which is mounted on the base 2 so as to cover the sensor section 3.
The sensor section 3, which is used as a passive sensor, for example, comprised of a pyroelectric element 3c for detecting quantity of energy of far infrared radiation from an object, detects changes of quantity of energy of far infrared radiation generated by movement of an individual who trespassed into a detection field 5, and a trespasser can be detected based on this change.
In the case of detector 1 having such a configuration, if an obstacle which disables the detection field 5 is intentionally added, such as placing an interfering cover which completely covers the cover 4, attaching a tape to the external surface of the cover 4, and coating the surface with paint, then the detection field 5 is blocked, disabling the detector 1 to detect a trespasser. To prevent this, in prior art, an obstacle detection section 6 is created near the detection field 5 (or inside the detector 1), an infrared light emitting element 3a emits light at every specified time, and the detection signals from an infrared light receiving element 3b are monitored.
Because of this structure, when the cover 4 is covered, a tape is attached to the external surface of the cover 4, or paint is coated on the surface, for example, in the detection field 5, detection signals from the infrared light receiving element 3b change. Thus the obstacle detection section 6 detects the above mentioned obstacles based on the change of reflected light quantity of infrared radiation from the infrared light emitting element 3a.
Such an obstacle detection method by the obstacle detection section 6, however, detects an obstacle outside the cover 4 from inside the cover 4. As a consequence, change of reflected light quantity of infrared radiation caused by an obstacle is small, and to detect this change of reflected light quantity, the obstacle detection section 6 must have a complicated circuit configuration which is of high cost.
There is another detector where a light receiving element is inside the cover and a light emitting element is outside the cover, near the detector main unit, so that an obstacle is detected when the obstacle blocks the direct optical path from the light emitting element to the light receiving element, but this method requires modification of the detector, therefore if the detector has already been installed, the detector must be replaced, and a detection area of a new detector must be readjusted after installation.
There is another detector in prior art, which was disclosed in U.S. Pat. No. 5,499,016, but the problem of this detector is that the detection field of the obstacle detection sensor is limited because the detector main unit is integrated with the obstacle detection sensor.